Charge Coupled Devices (CCDs) are well known detectors of optical radiation, typically visible radiation. A number of imaging devices and cameras have successfully employed CCD arrays to generate charge packets representing the optical energy arriving from a scene. FIG. 1 illustrates a conventional four phase CCD readout structure. By successively applying the proper potentials to a plurality of transfer gates (.phi.1-.phi.4) a given charge packet can be moved through a conductive transfer line 1, and thus from a location where the charge packet is created to a location where the charge packet is detected.
A so-called staring IR-FPA includes a two dimensional array of IR photodetectors that is arranged in a row and column matrix. The staring IR-FPA can be contrasted to a scanned-type of IR-FPA, wherein a rotating mirror or some similar mechanism scans a scene image across the IR-FPA. Incident IR radiation arriving from the scene is converted by each photodetector to charge and integrated within an associated discrete unit cell.
Referring to FIG. 2, in a typical embodiment of an N row by M column staring IR-FPA, each column of unit cells is provided with a column amplifier (CA), such as a capacitive transimpedance amplifier or CTIA. A first set of transistors (connected to a potential (V.sub.IN)) are controlled to properly bias the unit cell photodetector (PD), while a second set of transistors which function as switches (connected to timing signals transfer-phase or .phi.tr) are used to sequentially and individually connect N unit cells along a given column to a column sense or readout line 2 that is connected to the column's CA. The column CAs may read out all of the unit cells in parallel across a given one of the N rows, and then output a signal having a magnitude that is indicative of the IR radiation that was detected by the associated unit cell. This process continues until all N rows have been readout, thus forming an image frame. During an integration period, before readout, the detected current (I.sub.det) is integrated on a unit cell capacitance (C.sub.int). For a CTIA embodiment the column amplifier includes an integration capacitance and a solid state switch for periodically resetting the integration capacitance.
Due at least in part to the use of the CTIA, the circuitry of FIG. 2 will generally have a higher charge transfer efficiency (CTE) than the CCD detector, and is often preferred for low light level and thermal imaging applications.
Reference in this regard can be had, by example, to commonly assigned U.S. Pat. No.: 4,786,831, "Integrating Capacitively Coupled Transimpedance Amplifier", by Morse et al.; U.S. Pat. No. 4,956,716, "Imaging System Employing Charge Amplifier", by Hewitt et al.; U.S. Pat. No. 5,043,820, and "Focal Plane Array Readout Employing One Capacitive Feedback Transimpedance Amplifier for Each Column", by Wyles et al. The disclosures of these various patents are incorporated by reference herein in their entireties.
It is known in the art to operate a FPA in a Time Delay and Integrate (TDI) mode. By example, due to rotation of a platform that contains the FPA, a scene being viewed, such as a star, is periodically swept across the FPA. This results in a corresponding motion of the charge image across the FPA in a row-by-row fashion along a TDI "on-track" direction. Reference with regard to a TDI system can be had to, by example, commonly assigned U.S. Pat. No.: 5,453,781, "Apparatus and Method for Minimizing Velocity-Mismatch MTF Degradation in TDI Systems", by J. T. Stein. The disclosure of this U.S. Patent is incorporated by reference herein in its entirety.
It is known in the art to employ a CCD-type of visible light imaging array to view a moving scene. Reference in this regard can be had to U.S. Pat. No.: 5,155,597, "Electro-Optical Imaging Array with Motion Compensation", by Lareau et al. This patent describes an array having photosensitive cells that are arranged into one or more column groups. Charge packets collected in the cells are transferred down a column at the same rate as image motion in the plane of the array. Each column group may have its own charge transfer rate corresponding to the image motion rate of that column group. FIGS. 6 and 7 of this patent illustrate an isolated column group, and show the use of a three phase clock that is provided to all rows of the column group. Reference in this regard can also be had to the following two publications: "Electro-optical imaging array with motion compensation", SPIE Vol. 2023 Airborne Reconnaissance XVII (1993), pgs. 65-79, by A. G. Lareau; and "E-O framing camera flight test results", SPIE Vol. 2272 Airborne Reconnaissance XVIII (1994), pgs. 133-141, by A. G. Lareau and M. R. Brown.
It can be appreciated that it would be desirable to be able to image a scene in the IR radiation band with a staring type of FPA, wherein the scene is moving with resect to the IR-FPA, and to provide image motion compensation using a CCD. The prior art discussed above does not adequately address this need.